A process for coating fabric tubes is known from German Auslegeschrift No. 1,504,690, in which polymeric solutions are initially doctored onto the fabric in a thin layer and the coatings are subsequently dried by heating the tube. This process does, however, have two serious disadvantages. The tube has to be heated from the inside in order to impart a bubble-free surface to the fabric coating and this means an additional outlay on apparatus. Moreover, the presence of solvents makes processing more difficult since numerous measures have to be taken to suck off the solvent vapors, and processing plants which are protected from explosion are needed.
A process for coating fabric tubes with solvent-free systems (application of a polymer melt) is described in German Pat. No. 1,778,877. A disadvantage of this process is the high temperatures of about 200.degree. C. needed for processing thermoplastic materials. The melt bath, pipe work and stripping apparatus have to be kept at this high temperature level. In addition, the deeper penetration of the plastic coating into the fabric induced by the process causes marked stiffening of the material so that the finished tube becomes more difficult to handle.
Solvent-free reactive polyurethane compositions for coating fabrics, which are stable in storage, are described in German Offenlegungsschrift No. 1,570,48 and U.S. Pat. No. 3,475,200 and are composed of polyhydroxyl compounds, uretdione diisocyanates which melt above 100.degree. C. and chain extenders which melt above 80.degree. C. Although coating pastes of this type are stable in storage over relatively long periods of time, they need uneconomically long reaction times (for example, 90 minutes at 110.degree. C.) in order to cure completely.
The object of the invention is therefore to avoid the disadvantages described above of the known processes for coating fabrics--presence of solvent; too short pot-life; high processing temperatures and long curing times and simultaneously to allow simple and problem-free handling of the coating compositons.
This object is achieved by the polyurethane systems provided according to the invention.